1. Field of the Invention
The present invention relates to magnetic sensors and, more particularly, to a Hall-effect sensor arrangement having temperature compensation circuitry that is user-adjustable.
2. Discussion of Related Art
Hall-effect elements are used to sense magnetic fields. In its simplest form, a monolithically fabricated Hall-effect element employs a square plate formed, for example, of an epitaxial (epi) pocket formed in a bipolar (BP) or bipolar complementary metal oxide semiconductor (BiCMOS) process. The square plate has two pairs of contacts oriented such that each pair of contacts straddles the plate from a different pair of its diametrically opposing comers. When a current I.sub.H is caused to flow between one of the two pairs of contacts, a voltage V.sub.H will appear across the other pair of contacts. The voltage V.sub.H is a function of: (a) the amplitude of the current I.sub.H, (b) the strength of a magnetic field B, if any, intercepting the Hall plate, and (c) the sensitivity S.sub.H of the Hall plate to the magnetic field B (i.e., V.sub.H =I.sub.H *B*S.sub.H, where * denotes the multiplication operator). Thus, when properly calibrated, a Hall plate may be used to measure the strength of a magnetic field B by applying a known current I.sub.H between one pair of contacts of a Hall plate and measuring the voltage V.sub.H across the other pair of contacts.
There are two major difficulties encountered when using Hall plates to sense magnetic fields. One difficulty arises because a Hall plate includes unavoidable imperfections, such as geometrical asymmetries, that cause the Hall plate to produce a non-zero output voltage V.sub.H in the absence of any applied magnetic field B. This voltage V.sub.H generated when B=0 commonly is referred to as the offset voltage V.sub.OH of the Hall plate. A discussion of the Hall effect, the operation of Hall-effect elements, circuits in which Hall-effect elements may be used, and various techniques for addressing the problem of Hall plate offset voltage are discussed in the following patents and publications, each of which is incorporated herein by reference: Bilotti et al., U.S. Pat. No. 5,621,319; Mehrgardt et al., U.S. Pat. No. 5,406,202; Bilotti, Albert, "Monolithic Magnetic Hall Sensor Using Dynamic Quadrature Offset Cancellation," IEEE Journal of Solid-State Circuits, Vol. 32, No. 6, June 1997; Bellekom, A. A., and Munter, P. J. A., "Offset Reduction in Spinning-Current Hall Plates," Sensors and Materials, 5, 5 (1994) 253-263, MYU Tokyo; Baltes, H. P., and Popovic, R. S., "Integrated Semiconductor Magnetic Field Sensors," Proc. IEEE, vol. 74, pp. 1107-1132, August 1986.
A second difficulty arises due to the fact that the sensitivity S.sub.H of a Hall-effect element changes as the temperature of the element changes. Therefore, for a given current I.sub.H and magnetic field B, the voltage V.sub.H produced by a Hall-effect element will vary depending on the temperature of the element. Because of this temperature-dependency, the actual magnitude of the magnetic field B is difficult to ascertain when a Hall-effect element is used in an environment wherein its temperature is not held constant.
In addition, Hall-effect elements often are used to sense the permanent magnetic fields of components. For example, a Hall-effect element may be used to sense the position of a magnetic component while it is rotating or otherwise in motion relative to the Hall-effect element. Because the magnitude of a magnetic field emanating from a magnetic component changes as the temperature of the component changes, for any given position of the component with respect to the Hall-effect element, the magnitude of the output voltage V.sub.H of the Hall-effect element when the component is at a first temperature will not be the same as the output voltage V.sub.H when the component is at a second temperature. Therefore, the magnitude of the output voltage V.sub.H of a Hall-effect element does not provide a reliable indication of the position of a magnetic component when the component is subjected to a variable-temperature environment.
It therefore is a general object of the present invention to provide a Hall effect device having improved immunity to the effects of a variable-temperature environment.